1. Field of the invention
The present invention relates to an indirect conversion type radiation image detection device and a radiation imaging system having a radiation image detection device.
2. Description Related to the Prior Art
In a medical field or the like, a radiation imaging system is commonly used for observing the interior of a body cavity. The radiation imaging system comprises a radiation source and a radiation image detection device. The radiation source emits radiation such as X-rays to a subject. The radiation image detection device converts the radiation, which has passed through the subject, into a charge and then converts the charge into a voltage. Thereby the radiation image detection device produces image data, which represents a radiation image of the subject.
There are direct conversion type radiation image detection devices and indirect conversion type radiation image detection devices. The direct conversion type radiation image detection device directly converts the radiation into the charge. The indirect conversion type radiation image detection device converts the radiation into light (visible light) and then converts the light into the charge. The indirect conversion type radiation image detection device has a wavelength converting layer and a solid state detector. The wavelength converting layer converts the radiation into the light. The solid state detector converts the light, which has been generated by the wavelength converting layer, into the charge. The solid state detector has a plurality of photodiodes.
The wavelength converting layer contains phosphor for converting the radiation into the light. The phosphor is composed of particles (hereinafter referred to as phosphor particles) such as GOS(Gd2O2S:Tb) or columnar crystals such as CsI:Tl. A wavelength converting layer of particle structure is commonly used because it is easier to manufacture and less expensive than that of columnar crystal structure. The wavelength converting layer of “particle structure” refers to a wavelength converting layer in which phosphor particles are dispersed in a binder such as resin.
The indirect conversion type radiation image detection device has the wavelength converting layer and the solid state detector stacked together, one on top of the other. There are two types of the indirect conversion type radiation image detection devices, depending on which one of the wavelength converting layer and the solid state detector is disposed close to the radiation source. The radiation image detection device with the wavelength converting layer close to the radiation source is referred to as PSS (Penetration Side Sampling) type. The radiation image detection device with the solid state detector close to the radiation source is referred to as ISS (Irradiation Side Sampling) type (See Japanese Patent Laid-Open Publication No. 2010-112733).
The wavelength converting layer emits the light in response to the radiation incident thereon. The light is mainly generated in a surface layer on which the radiation is incident. Hence, in the PSS type, the light is mainly generated in the surface layer on the opposite side of the solid state detector. The generated light propagates through the wavelength converting layer to the solid state detector. During the propagation, part of the light is absorbed by the wavelength converting layer or scattered. As a result, sensitivity (conversion efficiency for converting radiation into light) and sharpness of an image detected by the solid state detector are reduced.
In the ISS type, the radiation, which has passed through the solid state detector, is incident on the wavelength converting layer. Hence, the light is generated on a solid state detector side of the wavelength converting layer. The ISS type has advantages that the light propagation distance is short and thereby the reduction of the sensitivity and sharpness is suppressed.
To improve the sensitivity of the wavelength converting layer of the ISS type, the thickness of the wavelength converting layer is increased, for example. However, this increases a distance between the phosphor particles in the wavelength converting layer and the solid state detector. The phosphor particles generate the light at positions away from the solid state detector. The light from the phosphor particles spreads out significantly as the light propagates to the solid state detector, resulting in reduction of the image sharpness. The sensitivity of the wavelength converting layer is improved by increasing the size of the phosphor particles and thereby increasing the light emission amount of the phosphor particles. However, this further spreads out the light, which is emitted from the phosphor particles and propagates to the solid state detector. As a result, the sharpness is further reduced.
The Japanese Patent Laid-Open Publication No. 2010-112733 suggests a wavelength converting layer composed of a first phosphor layer, in which phosphor particles with a small average particle diameter are dispersed in a binder, and a second phosphor layer, in which phosphor particles with a large average particle diameter are dispersed in a binder. The first and second phosphor layers are layered or stacked together. The second phosphor layer is disposed on the solid state detector side. The second phosphor layer has the large-sized phosphor particles and high light emission amount, and is located close to the solid state detector. Hence, the spreading or scattering of the light is small and the reduction of the sharpness is prevented. In the first phosphor layer, the phosphor particles are away from the solid state detector, but the size of the phosphor particles is small. Hence, the spreading or scattering of the light is small and the reduction of the sharpness is prevented. As a result, the radiation image detection device is capable of improving the sensitivity without the reduction in sharpness.
However, in the radiation image detection device according to the Japanese Patent Laid-Open Publication No. 2010-112733, the wavelength converting layer has a two-layer structure, which is composed of the first and second phosphor layers, to improve the sensitivity and the sharpness. The two-layer structure increases manufacturing cost. For this reason, improvements of the sensitivity and the sharpness with the use of a single wavelength converting layer is desirable.